Edman degradation chemistry with the formation of phenylthiohydantoin (PTH) derivatives of amino acids is the key to amino acid sequencing analyses of peptides by direct analysis. One of the limitations of the Edman degradation approach is the detectability of the PTH derivatives of the amino acid in the final step of the analysis. If greater sensitivity could be achieved at the detection level of the PTH derivative, the sample size requirements for amino acid sequence information from this technology could be reduced. Because of the high ionization efficiency available through electrospray (ESI), this technique would seem to be a good candidate for combination with the classical Edman chemistry sequenator. However, ionization of PTH derivatives of most amino acids by ESI is disappointly low. The goal of this project has been to prepare modified PTH derivative of amino acids that has an additional functional group such as an amino group which will provide a basic site for protonation during ESI. We have demonstrated that aminophenylisothiocyanate can be used successfully to derivatize amino acids to prepare an amino modified PTH derivative that substantially enhances the detectability of the amino acid derivative following cleavage from the peptide during the Edman procedure.